diff -u b/core/modules/filter/filter.module b/core/modules/filter/filter.module
--- b/core/modules/filter/filter.module
+++ b/core/modules/filter/filter.module
@@ -444,11 +444,13 @@
  * @param string $text
  *   The text to be filtered.
  * @param string $format_id
- *   The machine name of the filter format to be used to filter the text.
+ *   The machine name of the filter format to be used to filter the text. If
+ *   NULL, the fallback format is used. See filter_fallback_format().
  * @param string $langcode
  *   (optional) The language code of the text to be filtered, e.g. 'en' for
  *   English. This allows filters to be language-aware so language-specific text
- *   replacement can be implemented. Defaults to 'und'.
+ *   replacement can be implemented. Defaults to
+ *   Drupal\Core\Language\LanguageInterface::LANGCODE_NOT_SPECIFIED.
  * @param array $filter_types_to_skip
  *   (optional) An array of filter types to skip, or an empty array (default)
  *   to skip no filter types. All of the format's filters will be applied,